I saw that this app, PowerSketch is available for free for a short period of time. Grab it before it returns to normal pricing.
From the app's description...
Turn life's precious moments into artistic sketches in seconds
PowerSketch is a fascinating yet easy-to-use app that brings out the Picasso in you. 10+ realistic and creative sketch effects come standard with PowerSketch, including rice paper, picture-in-picture, shadow, wash, neon, and more. You can capture an image and turn it into a sketch or select an image from your camera roll or gallery to convert to a sketch.
First seen via Lifehacker.com's Today's App Deals
Here is an example of the charcoal sketch filter in the PowerSketch app.